Small wheels road bike, FC and trail targets ?

[christo]

Bonjour all
i am looking for some thoughts from experienced builders on the following : the next project is a road bike for my wife. I am looking for comfort and easy recreationnal rides. I know how she will be positionned (ie i have a good idea of saddle height, setback and handlebar position), but here is the difficulty : I will use 26" rims with 28m tyres. This is because my wife is only 1.63m, so building around small wheels is certainly easier. I won't use 650B wheels, as 26" rims are easier to find around here.

Again, positionning her is not the difficulty. I'll use a 73° seat tube angle, 260 mm BB height (170mm cranks). But I know struggle on the front end of the design, as i don't know what would be the target in terms of Front Center, HT angle and mechanical trail. For small wheels (609 mm external diameter in this case), should i reduce the trail (this bike won't carry front load) compared to a "normal" 700C ? If yes, down to what limit ? Should I play with HT angle and fork rake to get a "normal" FC (in the 580+ area) ? I am looking for an easy going bike, so what would be a comfortable HTA/Rake combination ?

As you can see, I don't really know where I'm heading to, so all your thoughts are more than welcome

Christophe

PS : needless to say that this bike will be for my wife, thus a non perfect geometry is absolutely not acceptable

[David Tollefson]

I built this all-road bike for my wife, designed around 24" wheels (Velocity Aeroheat 507 BCD rims). Front geometry was based on her bar position, "normal"* trail, and weight distribution (biased to the rear on this bike). Point being, the wheel size really only changed the BB drop, fork offset and fork/headtube length.

15023202039_f2202b3640_z.jpg

*"Normal" trail was on the high side for less responsive steering reaction -- she's not an avid rider.

[christo]

David, thak you for your answer. Could you clarify 2 points please :
1 - on the front end side,do you mean that mechanical trail should not be function on the wheel size, thus I should target the same value as I would with a 700C (ie in the 60-65mm range for a less responsive bike) ?
2 - about Front Center (ie mass distribution) I get your point when I look at your picture where the chainstays are very long. In my case, I intend to get rather short stays (in the 370mm range), so the mass distribution will be affected. Compared to a 700C, if I reduce the stays by a factor of .9, shouldn't I do the same for the FC ? If I do so, I would land in the 540mm range

Cheers
Christophe

[David Tollefson]

Those stays look long because of the small wheel size -- in reality they're a very normal 425mm.

As for the "should" part of the trail consideration, I can only say that's what I did, and it rides very nicely. I wanted the weight distribution to be as close to 60% on the rear wheel as possible, so the front-center was based on that, and the resultant fork offset and HTA from that wheel location and the bar position. Some one who has a lot more experience with small-wheeled bikes (Aimar comes to mind) might have more input.

[christo]

Might be easier with a little drawing : that is the status so far. As Doug Fattic said in a previous post, "The most likely 2nd frame someone will build is for a female companion. At least that is what many of my framebuilding class students say. Get it right and she will love you more and support your future framebuilding ambitions. Get it wrong and there will be bad consequences all around.". I can't screw up on this one, because this will be frame#9, and to be honest frame #2 was built for my wife and is a terrible bike. Therefore any advise or comment is more than welcome, I won't have a third chance.

Muriel 2015.jpg

[Pritchett]

Photos of the layout of my wife's 650b allroad-ish bike. It's also her commuter and occasionally carries a small load up front. I used a fork from a Surly 26" disc trucker. Relevant Geo is visible on the screen shot. Wife is 5'4" and she's very comfortable for long hauls on this bike and really likes the geo. I can't seem to find any good pics of her riding the things, but it is very proportionate looking.

I went 650bx38 so she could have a comfy ride with no toe overlap issues. I started with the handling geo I thought was appropriate, then shortened up the front center length as much as I was comfortable. Stem length was considered to make sure things weren't getting way out of whack. Rear end length was left a little long for more comfort. BB is very low. That's how we do at my house.

Kati Bike.JPGkati geo.JPG